As much as I respect GNUStep, I think Enlightenment is a better choice if we truly want a Linux DE on par with Aqua. Unless, of course, you're referring to easy cross-compiling of apps. In that case, yes GNUStep would be a logical choice. But I honestly think E17 is closer to a "production ready" state, as well as being more modern and (dare I say it) prettier.

Also, GNUStep is more than just a DE, it's an entirely different way of approaching desktop computing than the normal Linux model. You can put Windowmaker on any Linux distro and it's just a look-and-feel thing; spin up the GNUStep live CD and you get a better idea of how great the whole GNUStep philosophy is.

How about this: Finish both, put them on top of Debian Squeeze and see which one comes out on ahead. I'd root for either one myself.